This sounds like one of the Direct3D quirks - if you create a d3d device on a window on one monitor, then move the window to another monitor, things start running really slowly. Sublime tries to detect this and recreate the device when it's moved from one monitor to another but perhaps that's not working correctly in this case.
Can I get you to:
* Check that the window isn't spanning multiple monitors, this will always cause a slowdown.
* Move the window onto your 2ed display, exit, then restart, and see if it's still slow. This will create the window directly on the correct monitor, and so won't rely on any logic to detect a change of monitor.
If neither of these clear it up, I'll make the OpenGL renderer a command line option for the next beta, which doesn't (at least for me) have any slow down issues associated with moving windows between monitors, spanning monitors, etc.